Light-emitting device and method for manufacturing the same

ABSTRACT

A light-emitting element is disclosed that can drive at a low driving voltage and that has a longer lifetime than the conventional light-emitting element, and a method is disclosed for manufacturing the light-emitting element. The disclosed light-emitting element includes a plurality of layers between a pair of electrodes; and at least one layer among the plurality of layers contains one compound selected from the group consisting of oxide semiconductor and a metal oxide, and a compound having high hole transportation properties. Such the light-emitting element can suppress the crystallization of a layer containing one compound selected from the group consisting of oxide semiconductor and a metal oxide, and a compound having high hole transportation properties. As a result, a lifetime of the light-emitting element can be extended.

TECHNICAL FIELD

The present invention relates to a light-emitting element formed to havethe structure in which a plurality of layers is sandwiched between apair of electrodes. More specifically, the present invention relates tothe structure of a layer, which can be used as at least one layer,within the plurality of layers.

BACKGROUND ART

A light-emitting device utilizing light emission from anelectroluminescent element (light-emitting element) has been attractedattention as a display device or a lighting device.

As a light-emitting element used for a light-emitting device, alight-emitting element including a layer containing light-emittingcompounds interposed between a pair of electrodes is well known.

Within such a light-emitting element, either of the electrodes serves asan anode, and another serves as a cathode, holes injected from the anodeand electrons injected from the cathode are recombined with each otherto form molecular exciton, and the molecular exciton radiates energy aslight while returning to the ground state.

The demand for low power consumption is especially increased in displaydevices to be installed in various information processing devices, whichhave been drastically developed in recent years. In order to achieve lowpower consumption, it has been attempted to reduce a voltage for drivinga light-emitting element. In consideration with commercialization, it isimportant not only to reduce a voltage for driving a light-emittingelement but also extend a lifetime of a light-emitting device.Therefore, a light-emitting device has been developed to achieve lowpower consumption and a long lifetime.

For example, unexamined patent publication No. 9-63771 discloses that avoltage for driving a light-emitting element is reduced by forming ananode by a metal oxide having a large work function such as a molybdenumoxide. Moreover, a lifetime of the light-emitting element can beextended.

The means for extending a lifetime of a light-emitting element disclosedin the unexamined patent publication No. 9-63771 is insufficient, and soit is required to develop the technique for achieving a further longlifetime.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a light-emittingelement that drives at low voltage and has a longer lifetime than theconventional light-emitting element, and a method for manufacturing thelight-emitting element.

According to one aspect of the present invention, a light-emittingelement includes a plurality of layers between a pair of electrodes, andat least one layer among the plurality of layers contains a hybridmaterial (a composite material) including an inorganic compound selectedfrom the group consisting of oxide semiconductor and a metal oxide, andan organic compound having high hole transportation properties.

The plurality of layers is composed of layers formed respectively by asubstance having high carrier injection properties, a substance havinghigh carrier transportation properties, and the like, so that alight-emitting region is located away from electrodes.

Such the light-emitting element can suppress crystallization of a layercontaining one compound selected from the group consisting of oxidesemiconductor and a metal oxide, and a compound having high holetransportation properties. As a result, a lifetime of a light-emittingelement can be extended.

As specific examples of the oxide semiconductor and the metal oxide,although not limited to the substances recited herein, a molybdenumoxide (MoOx), a vanadium oxide (VOx), a ruthenium oxide (RuOx), atungsten oxide (WOx), and the like can be used. Besides, an indium tinoxide (ITO), a zinc oxide (ZnO), and a tin oxide (SnO) can be used.Other substances than the foregoing substances can be used.

As the compound having high hole transportation properties, although notlimited to the substances recited herein,4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviated α-NPD), anaromatic amine compound (that is, the one having a bond of benzene ringand nitrogen) such as4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (abbreviated TPD),4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviated TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(abbreviated MTDATA) can be used. Other substances than the foregoingsubstances can be used.

According to one aspect of the present invention, a light-emittingelement includes a plurality of layers between a pair of electrodes, andat least one layer among the plurality of layers contains one compoundselected from the group consisting of oxide semiconductor and a metaloxide, a compound having high hole transportation properties, and acompound having a high steric hindrance.

As same as the aforementioned light-emitting element, the plurality oflayers is composed of layers respectively formed by a substance havinghigh carrier injection properties, a substance having high carriertransportation properties, and the like, so that a light-emitting regionis located away from electrodes.

Such the light-emitting element that includes a layer containing onecompound selected from the group consisting of oxide semiconductor and ametal oxide, a compound having high hole transportation properties, anda compound having a high steric hindrance can suppress thecrystallization of the layer. As a result, a lifetime of alight-emitting element can be extended.

The oxide semiconductor, the metal oxide, and the compound having highhole transportation properties are the same as those mentioned above.

As a compound having a high steric hindrance (that is, the coupoundhaving spatiality contrary to a plane structure),5,6,11,12-tetraphenyltetracene (abbreviated rubrene) is preferably used.Besides, hexaphenylbenzene diphenylanthracene, t-butylperylene,9,10-di(phenyl)anthracene, coumarin 545T, and the like can be used. Inaddition, dendrimer or the like can be used.

According to the present invention, aggregation of oxide semiconductoror a metal oxide can be suppressed, and crystallization of a layercontaining the oxide semiconductor or the metal oxide can be suppressed.By suppression of the crystallization, a leak current can be preventedfrom generating due to the crystallization. Accordingly, a long-livedlight-emitting element can be obtained.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading of the followingdetailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph for showing luminance variations with time oflight-emitting elements according to certain aspects of the presentinvention and Comparative Example;

FIGS. 2A to 2C are explanatory cross-sectional views of a light-emittingelement according to certain aspects of the present invention;

FIG. 3 is an explanatory cross-sectional view of a light-emittingelement according to certain aspects of the present invention;

FIG. 4 shows luminance-voltage characteristics of light-emittingelements according to certain aspects of the present invention andComparative Example 1;

FIGS. 5A to 5C are cross-sectional views of a light-emitting elementaccording to certain aspects of the present invention;

FIG. 6 is a top view of a light-emitting device including alight-emitting element according to certain aspects of the presentinvention;

FIG. 7 is a view for showing an electric appliance mounted with alight-emitting device including a light-emitting element according tocertain aspects of the present invention;

FIG. 8 shows luminance-voltage characteristics of light-emittingelements according to certain aspects of the present invention andComparative Example 1; and

FIG. 9 is a graph for showing luminance variations with time of alight-emitting element according to certain aspects of the presentinvention.

FIG. 10 shows luminance-voltage characteristics of light-emittingelements according to certain aspects of the present invention.

FIG. 11 is a graph for showing luminance variations with time of alight-emitting element according to certain aspects of the presentinvention.

FIG. 12 is a graph for showing voltages for electroluminescence of 1cd/m² or more according to certain aspects of the present invention andComparative Example 2.

FIG. 13 is a graph for showing current density-voltage characteristicsof light-emitting elements according to certain aspects of the presentinvention.

FIG. 14 is a graph for showing current density-voltage characteristicsof light-emitting elements according to certain aspects of ComparativeExample 3.

FIG. 15 is a graph for showing current density-voltage characteristicsof light-emitting elements according to certain aspects of ComparativeExample 4.

FIG. 16 is a graph for showing absorption characteristics oflight-emitting element according certain aspects of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

A light-emitting element according to the present invention includes aplurality of layers between a pair of electrodes. The plurality oflayers is formed by stacking layers in combination with each other,which is formed respectively by a substance having high carrierinjection properties or a substance having high carrier transportationproperties, so that a light-emitting region is formed away from theelectrodes, that is, carriers are recombined with each other in theregion away from the electrodes.

One mode of a light-emitting element according to the present inventionis explained with reference to FIG. 2A.

In this embodiment, a light-emitting element 210 is formed over asubstrate 201 used for supporting the light-emitting element 210, and iscomposed of sequentially a first electrode 202, a first layer 203, asecond layer 204, a third layer 205, a fourth layer 206, and a secondelectrode 207. Further, the first electrode 202 serves as an anode, andthe second electrode 207 serves as a cathode in this embodiment.

As the substrate 201, for example, glass or plastic can be used. Anothermaterial can be used for the substrate as long as it serves as a supportmedium for the light-emitting element during a manufacturing process.

The first electrode 202 is preferably formed by a metal having a largework function (at least 4.0 eV), an alloy, an electric conductivecompound, or a mixture of the foregoing materials. Specifically, anindium tin oxide (ITO), an indium tin oxide containing silicon, anindium zinc oxide, that is, an indium oxide mixed with zinc oxide offrom 2 to 20% (ZnO), aurum (Au), platinum (Pt), nickel (Ni), tungsten(W), chromium (Cu), molybdenum (Mo), ferrum (Fe), cobalt (Co), copper(Cu), palladium (Pd), or a nitride of a metal material (TiN), or thelike can be used.

The first layer 203 is a layer containing a compound selected from thegroup consisting of oxide semiconductor and a metal oxide, and acompound having high hole transportation properties. As specificexamples of the oxide semiconductor and the metal oxide, although notlimited to the substances recited herein, a molybdenum oxide (MoOx), avanadium oxide (VOx), a ruthenium oxide (RuOx), a tungsten oxide (WOx),and the like can be used. Other substances than the foregoing substancescan be used. As a compound having high hole transportation properties,for example, an aromatic amine compound (that is, the one having a bondof a benzene ring and nitrogen) such as4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (abbreviated α-NPD),4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (abbreviated TPD),4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviated TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(abbreviated MTDATA) can be used. The foregoing substances are mainlythe substances having hole mobility of 10⁻⁶ cm²/Vs or more. Anothersubstance can be used as long as it has higher hole transportationproperties than electron transportation properties.

The first layer 203 having the structure as mentioned above has highhole injection properties. In the first layer 203, the aggregation ofoxide semiconductor or a metal oxide is suppressed by a substance havinghigh hole transportation properties contained in the layer. That is,crystallization of the first layer 203 is suppressed. In addition, thefirst layer 203 may be formed by, for example, at least two layersrespectively containing semiconductor and a compound having hightransportation properties with different mixture ratio from each otherinstead of being formed by one layer.

Further, the first layer 203 may further includes a compound having ahigh steric hindrance (that is, the compound having spatiality contraryto a plane structure) in addition to a compound selected from oxidesemiconductor and a metal oxide, and a steric hindrance,5,6,11,12-tetraphenyltetracene (abbreviated rubrene) is preferably used.Besides hexaphenylbenzene, diphenylanthracene, t-butylperylene,9,10-di(phenyl)antracene, coumarin 545T, and the like can be used. Inaddition, dendrimer or the like can be used. Thus, crystallization of amolybdenum oxide can be further suppressed by mixing a substance with astructure having a high steric hindrance, that is, having spatialitycontrary to a plane structure, into the first layer 203.

The second layer 204 is a layer formed by a substance having high holetransportation properties, for example, an aromatic amine compound (thatis, the one having a bond of a benzene ring-nitrogen) such as α-NPD,TPD, TDATA, MTDATA, and the like. The foregoing substances are mainlythe substances having hole mobility of 10⁻⁶ cm²/Vs or more. Anothersubstance can be used as long as it has higher hole transportationproperties than electron transportation properties. In addition, thesecond layer 204 may be formed by stacking at least two layerscontaining the foregoing substances instead of being formed by onelayer.

The third layer 205 is a layer containing a substance having highlight-emitting properties. For example, a substance having highlight-emitting properties such as N,N′-dimethylquinacridone (abbreviatedDMQd), 2H-chromene-2-on (abbreviated coumarin), and a substance havinghigh carrier transportation properties and a good film formationproperty, that is, a substance that is hard to be crystallized, such astris(8-quinolinolato)aluminum (abbreviated Alq₃) or9,10-di(2-naphthyl)anthracene (abbreviated DNA) can be used by freelycombining with each other. Alq₃ and DNA have high light-emittingproperties, consequently, the third layer 205 may be solely formed byAlq₃ or DNA.

The fourth layer 206 is a layer formed by a metal complex or the likehaving a quinoline skeleton or a benzoquinoline skeleton, for example, asubstance having high electron transportation properties such astris(8-quinolinolato)aluminum (abbreviated Alq₃),tris(5-methyl-8-quinolinolato)aluminum (abbreviated Almq₃),bis(10-hydroxybenz)[h]-quinolinato)beryllium (abbreviated BeBq₂), andbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviatedBAlq). Further, a metal complex having a oxadiazole ligand or a thiazoleligand such as bis[2-(2-hydroxyphenyl)-benzooxazolate]zinc (abbreviatedZn(BOX)₂), and bis[2-(2-hydroxyphenyl)-benzothiazolate]zinc (abbreviatedZn(BTZ)₂). Besides the metal complexes,2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviatedPBD), and 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviated OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviated TAZ), and3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviated p-EtTAZ), bathophenanthroline (abbreviated BPhen),bathocuproin (abbreviated BCP), and the like can be used. The foregoingsubstances are the substances having hole mobility of 10⁻⁶ cm²/Vs ormore. Another substance can be used as long as it has higher holetransportation properties than electron transportation properties. Inaddition, the fourth layer 206 may be formed by at least two layerscontaining the foregoing substances instead of being formed by onelayer.

As a material for the second electrode 207, metals having a small workfunction (at most 3.8 eV), alloys, electric conductive compounds, or amixture of the above are preferably used. As a specific example ofcathode materials, metals belonging to a 1 or 2 group of the periodictable of the elements, that is, alkali metals such as lithium (Li) orcesium (Cs); alkali earth metals such as magnesium (Mg), calcium (Ca),strontium (Sr); and alloys including the above elements (Mg:Ag, Al:Li)can be used. In case that a layer promoting electron injection isstacked between the second electrode 207 and a light-emitting layer,various conductive materials such as Al, Ar, ITO containing silicon, andthe like can be used regardless of their work function can be used forthe second electrode 207.

As the layer promoting electron injection, a compound of an alkali metalor an alkali earth metal such as lithium fluoride (LiF), cesium fluoride(CsF), calcium fluoride (CaF₂), and the like. Besides, a layer formed bya substance having electron transportation properties containing analkali metal or an alkali earth metal, for example, Alq₃ containingmagnesium (Mg), or the like can be used.

The first layer 203, the second layer 204, the third layer 205, and thefourth layer 206 may be formed by the method other than vapordeposition. For example, ink jetting, spin coating, and the like can beused. Each electrode or each layer may be formed by different filmformation methods, respectively.

Within a light-emitting element according to the invention having theforegoing structure, a current is flowed due to electric potentialdifferences generated between the first electrode 201 and the secondelectrode 207, and holes and electrons are recombined with each otherwithin the third layer 205, which is a layer containing highlight-emitting properties, then, light is generated. Therefore, alight-emitting region is formed in the third layer 205. However, theentire third layer 205 is not required to be served as a light emissionregion. For example, the light-emitting region may be formed either theside of the second layer 204 or the fourth layer 206 within the thirdlayer 205.

Light is emitted to the outside passing through either the firstelectrode 202 or the second electrode 207, or both of them. Therefore,either the first electrode 202 or the second electrode 207, or both ofthem are formed by a substance having light-transmitting properties. Inthe case that only the first electrode 202 is formed by a substancehaving light-transmitting properties, light is emitted from thesubstrate passing through the first electrode 202 as shown in FIG. 2A.In the case that only the second electrode 207 is formed by a substancehaving light-transmitting properties, light is emitted from the oppositeside of the substrate passing through the second electrode 207. In thecase that the first electrode 202 and the second electrode 207 areformed by substances having light-transmitting properties, light isemitted from both of the substrate and the opposite side of thesubstrate passing through the first electrode 202 and the secondelectrode 207.

The structure of a layer formed between the first electrode 202 and thesecond electrode 207 is not limited to the above described structure.The layer structure may have the other structure than the foregoing oneas long as the structure is formed to have an area for the recombinationof holes and electrons is located away from the first electrode 202 andthe second electrode 207 to prevent quenching due to that alight-emitting region and a metal are close to one another; and a layercontaining a compound selected from the group consisting of oxidesemiconductor and a metal oxide, and a compound having high holetransportation properties (moreover, a compound having a high sterichindrance may be contained). The lamination structure of the layer isnot restricted especially. A layer formed by a substance having highelectron transportation properties or hole transportation properties, asubstance having electron injection properties, a substance having holeinjection properties, a substance having bipolar properties (highelectron or hole transportation properties), or the like; and a layerformed by a compound selected from the group consisting of oxidesemiconductor and a metal oxide, and a compound having high holetransportation properties may be freely combined with each other.Further, an area for recombination of carriers may be controlled byproviding an ultra thin layer such as a silicon oxide layer. Forexample, the structure shown in FIG. 3 may be formed. However, a layerstructure is not limited thereto.

The light-emitting element shown in FIG. 3 is formed by stacking a firstelectrode 502 serving as a cathode; a first layer 503 formed by asubstance having high electron transportation properties; a second layer504 formed by a substance having high light-emitting properties; a thirdlayer 505 formed by a substance having high hole transportationproperties; a fourth layer 506 containing a compound selected from thegroup consisting of oxide semiconductor and a metal oxide, and acompound having high hole transportation properties; and a secondelectrode 507 serving as an anode. In addition, reference numeral 501shown in FIG. 3 denotes a substrate.

According to this embodiment, a light-emitting element is manufacturedover a glass or plastic substrate. A passive type light-emitting devicecan be manufactured by manufacturing a plurality of such light-emittingelements over one substrate. Instead of the glass or plastic substrate,a light-emitting element can be manufactured over a thin film transistor(TFT) array. Thus, an active matrix type light-emitting device thatcontrols the drive of a light-emitting element by a TFT can bemanufactured. The structure of the TFT is not especially restricted. TheTFT may be either a staggered TFT or an inversely staggered TFT. A drivecircuit formed over the TFT array substrate may be formed by both anN-type TFT and a P-type TFT. Alternatively, the drive circuit may beformed by either an N-type TFT or a P-type TFT.

Accordingly, the crystallization of the layer can be suppressed in thelight-emitting element including a layer containing a compound selectedfrom the group consisting of oxide semiconductor and a metal oxide, anda substance having high hole transportation properties. Therefore, thegeneration of a leak current due to the crystallization of the layer canbe suppressed, and a long-lived light-emitting element can be obtained.The crystallization of the layer can be further suppressed, and afurther long-lived light-emitting element can be obtained in thelight-emitting element including a layer containing a compound selectedfrom the group consisting of oxide semiconductor and a metal oxide, asubstance having high hole transportation properties, and a substancehaving a high steric hindrance.

Example 1

Hereinafter, a method for manufacturing a light-emitting elementaccording to the present invention and properties of the light-emittingelement is explained.

A first electrode is formed by depositing an indium tin oxide (ITO) overa glass substrate. Then, the glass substrate deposited with the ITO isprocessed in vacuum at 150° C. for 30 minutes.

A first layer is formed by co-evaporation of a molybdenum oxide andα-NPD having high hole transporting properties over the first electrode.The weight ratio of the molybdenum oxide and the α-NPD is 0.245:1. Thefirst layer is formed to have a thickness of 130 nm. As used herein, theterm “co-evaporation” refers to a method for evaporating each ofmaterials from a plurality of evaporation sources provided in oneprocessing chamber, and mixing the evaporated materials in the gas phaseto deposit the mixed materials onto a subject.

A second layer is formed by vapor deposition of α-NPD over the firstlayer to have a thickness of 10 nm.

A third layer is formed by co-evaporation of Alq₃ and coumarin-6 overthe second layer. The weight ratio of the Alq₃ and the coumarin-6 is1:0.002. The third layer is formed to have a thickness of 37.5 nm.

A fourth layer is formed by vapor deposition of Alq₃ over the thirdlayer to have a thickness of 37.5 nm.

A fifth layer is formed by vapor deposition of calcium fluoride (CaF₂)over the fourth layer to have a thickness of 1 nm.

A second electrode is formed by vapor deposition of aluminum over thefifth layer to have a thickness of 200 nm.

FIGS. 1 and 4 show device characteristics of thus manufacturedlight-emitting element.

FIG. 4 shows luminance-voltage characteristics of a light-emittingelement manufactured according to Example 1. In FIG. 4, a horizontalaxis shows an applied voltage (V), and a vertical axis shows luminance(cd/m²). FIG. 4 shows that an onset voltage, which is a voltage forelectroluminescence of 1 cd/m² or more, is approximately 2.5 V.

FIG. 1 shows a measurement result of the variation of luminance per hourof a light-emitting element manufactured according to Example 1. In FIG.1, a horizontal axis shows hour, and a vertical axis shows luminance.The luminance is represented its relative value to initial luminancebased on the initial value of 100. The variation of luminance ismeasured by applying continuously a constant amount of current to alight-emitting element, that is, applying a stress to a light-emittingelement; and the luminance of the light-emitting element is measured atgiven time intervals. A current that has a current density required toobtain electroluminescence with a luminance of 1000 cd/m² at initialstate is applied, and the luminance obtained at the applied current ismeasured. FIG. 1 shows that after 100 hours luminance becomes the valueof 82, that is, the luminance is decreased by 18% compared to theinitial luminance.

Example 2

A method for manufacturing a light-emitting element according to thepresent invention and characteristics of the light-emitting element isexplained.

A first electrode is formed by depositing an indium tin oxide (ITO) overa glass substrate. The glass substrate deposited with the ITO isprocessed in vacuum at 150° C. for 30 minutes.

A first layer is formed over the first electrode by co-evaporation of amolybdenum oxide, α-NPD having high hole transportation properties, andrubrene having a high steric hindrance. The weight ratio of themolybdenum oxide and the rubrene is 0.245:0.018. The first layer isformed to have a thickness of 130 nm.

A second layer is formed by vapor deposition α-NPD over the first layerto have a thickness of 10 nm.

A third layer is formed over the second layer by co-evaporation of Alq₃and coumarin-6 to have a thickness of 37.5 nm. The weight ratio of theAlq₃ and the coumarin-6 is 1:0.002.

A fourth layer is formed by vapor deposition of Alq₃ over the thirdlayer to have a thickness of 37.5 nm.

A fifth layer is formed by vapor deposition of calcium fluoride (CaF₂)over the fourth layer to have a thickness of 1 nm.

A second electrode is formed by vapor deposition of aluminum over thefifth layer to have a thickness of 200 nm.

FIGS. 1 and 4 show device characteristics of thus manufacturedlight-emitting element. The measurement method or the like is the sameas that described in Example 1.

FIG. 4 shows that an onset voltage, which is a voltage forelectroluminescence of 1 cd/m² or more, is approximately 2.5 V.

FIG. 1 shows that after 100 hours luminance becomes the value of 92,that is, the luminance is decreased by 8% compared to the initialluminance.

Example 3

A method for manufacturing a light-emitting element according to thepresent invention and properties of the light-emitting element isexplained.

A first electrode is formed by depositing an indium tin oxide (ITO) overa glass substrate. The glass substrate deposited with the ITO isprocessed in vacuum at 150° C. for 30 minutes.

A first layer is formed over the first electrode by co-evaporation of amolybdenum oxide,4,4-bis(N-(4-(N,N-di-m-tolylamino)phenyl)-N-phenylamino)biphenyl(abbreviated DNTPD) having high hole transportation properties, andrubrene having a high steric hindrance. The weight ratio of themolybdenum oxide, the DNTPD, and the rubrene is 0.5:1:0.05. The firstlayer is formed to have a thickness of 120 nm.

A second layer is formed by vapor deposition of α-NPD over the firstlayer to have a thickness of 10 nm.

A third layer is formed over the second layer by co-evaporation of Alq₃and coumarin-6 to have a thickness of 37.5 nm. The weight ratio of theAlq₃ and the coumarin-6 is 1:0.003.

A fourth layer is formed by vapor deposition of Alq₃ over the thirdlayer to have a thickness of 37.5 nm.

A fifth layer is formed by vapor deposition of calcium fluoride (CaF₂)over the fourth layer to have a thickness of 1 nm.

A second electrode is formed by vapor deposition of aluminum over thefifth layer to have a thickness of 200 nm.

FIGS. 8 and 9 shows device characteristics of thus manufacturedlight-emitting element. The measurement method or the like is the sameas that described in Example 1. In FIG. 8, a horizontal axis shows anapplied voltage (V), and a vertical axis shows luminance (cd/m²). InFIG. 9, a horizontal axis shows hour, and a vertical axis showsluminance (relative value).

FIG. 8 shows that an onset voltage, which is a voltage forelectroluminescence of 1 cd/m² or more, is approximately 2.4 V.

FIG. 9 shows that after 100 hours luminance becomes the value of 93,that is, the luminance is decreased by 7% compared to the initialluminance.

Comparative Example 1

As a comparative example, a method for manufacturing a light-emittingelement in which a second layer is formed by only a molybdenum oxide,and device characteristics of the light-emitting element are explained.

An indium tin oxide (ITO) is deposited over a glass substrate to form afirst electrode. The glass substrate deposited with the ITO is processedin vacuum at 150° C. for 30 minutes.

A first layer is formed by vapor deposition of a molybdenum oxide overthe first electrode to have a thickness of 100 nm.

A second layer is formed by vapor deposition of α-NPD over the firstlayer to have a thickness of 60 nm.

A third layer is formed over the second layer by co-evaporation of Alq₃and coumarin to have a thickness of 37.5 nm. The weight ratio of theAlq₃ and the coumarin is 1:0.002.

A fourth layer is formed by vapor deposition of Alq₃ over the thirdlayer to have a thickness of 37.5 nm.

A fifth layer is formed by vapor deposition of calcium fluoride (CaF₂)over the fourth layer to have a thickness of 1 nm.

A second electrode is formed by vapor deposition of aluminum over thefifth layer to have a thickness of 200 nm.

FIGS. 1 and 4 shows device characteristics of thus manufacturedlight-emitting element. The measurement method or the like is the sameas that described in Example 1.

FIG. 4 shows that an onset voltage, which is a voltage forelectroluminescence of 1 cd/m² or more, is approximately 2.5 V.

FIG. 1 shows that the luminance of a light-emitting element according tothe comparative example after 100 hours is drastically decreased, andlight emission cannot be generated. This is caused by the fact that aleak current is generated due to the crystallization of the secondlayer.

Accordingly, the light-emitting element described in Example 1 or 2emits light at the same driving voltage for the light-emitting elementdescribed in Comparative Example. As same as the light-emitting elementhaving a layer formed by only a molybdenum oxide, a light-emittingelement according to the invention is operated at a low voltage. Thelifetime of the light-emitting element described in Example 1 or 2 canbe extended. This arises from the fact that the crystallization ofP-type compounds is suppressed by being mixed with a compound havinghigh transportation properties such as α-NPD or a compound having a highsteric hindrance such as rubrene.

Example 4

An active matrix light-emitting device having a light-emitting elementaccording to the present invention is explained in Example 4.

As shown in FIG. 5, a drive transistor 91 and a light-emitting element92 according to the invention are provided over a substrate 90. Thedrive transistor 91 for driving a light-emitting element is electricallyconnected to the light-emitting element 92 according to the invention.As used herein, the term “light-emitting element” refers to a layerwhich includes a light-emitting layer between an electrode 93 and anelectrode 94, and which includes partly a layer containing a molybdenumoxide, a substance with high transportation properties, a substance witha high steric hindrance. The light-emitting element 92 is divided by abank layer 95.

A light-emitting element according to the invention may have thestructure in which an electrode serving as an anode is located at alower side (the term “lower” refers to the region where a drivetransistor is provided in response to the light-emitting element) and anelectrode serving as a cathode is located at an upper side (the term“upper” refers to the region where a drive transistor is not provided inresponse to the light-emitting element). Alternatively, thelight-emitting element may have the structure in which the electrodeserving as a cathode is located at the lower side, and the electrodeserving as anode is located at the upper side.

In the case of the former, the drive transistor 91 is a p-channel type,and in the case of the latter, the drive transistor 91 is an n-channeltype. The drive transistor 91 may be either a staggered type or aninversely staggered type. The semiconductor layer of the drivetransistor 91 may be any one of a crystalline semiconductor layer, anamorphous semiconductor layer, or a semiamorphous semiconductor layer.

The semiamorphous semiconductor has an intermediate structure between anamorphous structure and a crystalline structure (including singlecrystals and poly crystals). The semiamorphous semiconductor has astable third state with respect to free energy, and a crystalline regionhaving a short-range order and lattice distortion. At least a part ofthe semiconductor, crystal grains with grain diameters of from 0.5 to 20nm. A raman spectrum is shifted to a lower wave number than 520 cm⁻¹. ByX-ray diffraction, diffraction peaks (111), (220) that may be derivedfrom a Si crystalline lattice are observed. Hydrogen or halogen of 1atomic % or more is contained the semiconductor as neutralizer fordangling bond. Such semiconductor is referred to as what is called microcrystal semiconductor. A silicide gas is formed by glow dischargedecomposition (plasma CVD). As the silicide gas, Si₂H₆, SiH₂Cl₂, SiHCl₃,SiCl₄, SiF₄, or the like in addition to SiH₄ can be used. The silicidegas can be diluted by H₂, or the H₂ and one or a plurality of rare gaselements selected from the group consisting of He, Ar, Kr, and Ne. Thedilution rate is in the range of from 2 to 1000 times. An appliedvoltage is in the range of from 0.1 to 133 Pa. A power source frequencyis in the range of from 1 to 120 MHz, preferably, 13 to 60 MHz. A heattemperature for a substrate is at most 300° C., preferably, 100 to 250°C. As impurity elements in the semiconductor film, atmosphericconstituents such as oxygen, nitrogen, carbon, and the like havepreferably concentrations of at most 1×10²⁰/cm³, especially, oxygenconcentration is at most 5×10¹⁹/cm³, preferably, 1×10¹⁹/cm³. A TFT (thinfilm transistor) including semiamorphous semiconductor has mobility ofapproximately from 1 to 10 m²/Vsec.

Either the electrode serving as an anode or the electrode serving as acathode, or both of them are formed by a material havinglight-transmitting properties.

In case that an electrode serving as an anode is located at the lowerside, and only the electrode serving as an anode is formed by asubstance having light-transmitting properties; light is emitted throughthe side provided with a drive transistor as shown in FIG. 5A. Further,in case that an electrode serving as a cathode is located at the lowerside, and only the electrode serving as a cathode is formed by amaterial having light-transmitting properties, light is emitted throughthe side provided with a drive transistor as shown in FIG. 5A. In casethat an electrode serving as an anode is located at the lower side, andonly the electrode serving as a cathode is formed by a material havinglight-transmitting properties, light is emitted through the oppositeside provided with a drive transistor as shown in FIG. 5B. In case thatan electrode serving as a cathode is located at the lower side, and onlythe electrode serving as an anode is formed by a material havinglight-transmitting properties, light is emitted through the oppositeside provided with a drive transistor as shown in FIG. 5B. In case thatboth of an electrode serving as an anode and an electrode serving as acathode are formed by materials having light-transmitting properties,light is emitted from both sides as shown in FIG. 5C irrespective ofwhich electrode is provided to lower side or upper side.

The light-emitting element may emit either monochromatic light or lightin full color such as red (R), green (G), blue (B), and the like. Eachlight-emitting element is preferably divided by a bank layer. The banklayer may be formed by a material that contains either an inorganicsubstance or an organic substance, or both of the inorganic substanceand organic substance. For example, a silicon oxide film may be used. Amaterial having a skeleton structure formed by the bond of acrylic,polyimide, and (a substance in which a skeleton is formed by the bond ofsilicon (Si) and oxygen (O); and at least hydrogen is included as asubstituent, or at least one element selected from the group consistingof fluoride, alkyl group, and aromatic hydrocarbon is included as thesubstituent), or the like may also be used. Further, the bank layer ispreferably formed to have a curved edge portion whose radius ofcurvature is continuously varied.

Since a light-emitting element according to the present invention has along lifetime, a light-emitting device manufactured by practicing thepresent invention can display images and emit light in a long term.

Example 5

A light-emitting device including a light-emitting element according tothe present invention as shown in Example 4 is mounted on variouselectric appliances after attached with an external input terminal.

Such electric appliances manufactured by practicing the presentinvention can display high quality images in a long term. This arisesfrom the fact that a light-emitting element according to the inventionhas long lifetime.

In Example 5, a light-emitting device including a light-emitting elementand electric appliances mounted with the light-emitting element areexplained with reference to FIGS. 6 and 7.

FIG. 6 is a top view of a light-emitting device including alight-emitting element according to the present invention. FIG. 7 is oneembodiment of an electric appliance only, and the structure of alight-emitting device is not limited thereto.

In FIG. 6, reference numeral 401 indicated by dotted line denotes adrive circuit portion (a source side drive circuit); 402, a pixelportion; and 403, a drive circuit portion (a gate side drive circuit).Reference numeral 404 denotes a sealing substrate, and 405 denotes aportion applied with sealing agent.

Signals are inputted to the source side drive circuit 401 and the gateside drive circuit 403 when these circuits receive video signals, clocksignals, start signals, reset signals, and the like via wirings providedto a device substrate 410 from an FPC (flexible print circuit) 409serving as an external input terminal. Despite illustrated only the FPCin FIG. 6, the FPC may be provided with a printed wiring board (PWB). Alight-emitting device according to Example 5 refers to not only a mainbody of a light-emitting device, but also a main body provided with anFPC or a PWB.

FIG. 7 is one embodiment of an electric appliance installed with alight-emitting device as shown in FIG. 6.

FIG. 7 shows a laptop computer manufactured by practicing the presentinvention composed of a main body 5501, a housing 5502, a displayportion 5503, a keyboard 5504, and the like. A display device can becompleted by incorporating a light-emitting device including alight-emitting element according to the invention into the laptopcomputer.

Despite explained a laptop computer in Example 5, a cellular phone, a TVreception set, a car navigation, a lighting system, and the like may beinstalled with a light-emitting device including a light-emittingelement according to the invention.

Example 6

A method for manufacturing a light-emitting element according to thepresent invention, and device characteristics of the light-emittingelement are explained in Example 6. In Example 6, four light-emittingelements, that is, a light-emitting element (1), a light-emittingelement (2), a light-emitting element (3), and a light-emitting element(4), all of which have first layers with different thicknesses, aremanufactured.

A first electrode is formed by an indium tin oxide (ITO) containing asilicon oxide over a glass substrate. Then, the glass substrate providedwith the first electrode is treated in vacuum at 150° C. for 30 minutes.

A first layer is formed by co-evaporation of a molybdenum oxide,rubrene, and DNTPD over the first electrode to have a weight ratio of0.67:1:0.02, respectively. The first layer of a light-emitting element(1) is formed to have a thickness of 40 nm. The first layer of alight-emitting element (2) is formed to have a thickness of 80 nm. Thefirst layer of a light-emitting element (3) is formed to have athickness of 120 nm. The first layer of a light-emitting element (4) isformed to have a thickness of 160 nm.

A second layer is formed by vapor deposition of α-NPD over the firstlayer to have a thickness of 10 nm.

A third layer is formed by co-evaporation of Alq₃ and coumarin 6 overthe second layer to have a weight ratio of 1:0.005. The third layer isformed to have a thickness of 37.5 nm.

A fourth layer is formed by vapor deposition of Alq₃ over the thirdlayer to have a thickness of 37.5 nm.

A fifth layer is formed by vapor deposition of lithium fluoride (LiF)over the fourth layer to have a thickness of 1 nm.

A second electrode is formed by vapor deposition of aluminum over thefifth layer to have a thickness of 200 nm.

FIGS. 10 and 11 show the device characteristics of thus manufacturedlight-emitting elements.

FIG. 10 shows luminance-voltage characteristics of light-emittingelements manufactured according to Example 6. In FIG. 10, a horizontalaxis shows voltage (V), and a vertical axis shows luminance (cd/m²). Inaddition, closed circles indicate characteristics of the light-emittingelement (1); open circles indicate characteristics of the light-emittingelement (2); closed squares indicate characteristics of thelight-emitting element (3); and open squares indicate characteristics ofthe light-emitting element (4). FIG. 10 shows that an onset voltage,which is a voltage for electroluminescence of 1 cd/m² or more, isapproximately 2.5 V in each light-emitting element. The onset voltagesare approximately the same with each other despite the thickness of thefirst layer of each the light-emitting element. That is, the onsetvoltage for a light-emitting element practiced by the present inventionis insensitive to the thickness of the first layer. Therefore, externalcoupling efficiency of light emission from a light-emitting element inwhich an electrode is used to reflect light emission becomes easilyimproved by controlling an optical path length by variations of thethickness of a first layer according to the present invention.

FIG. 11 shows a measurement result of the variation of luminance perhour of a light-emitting element manufactured according to Example 6. InFIG. 11, a horizontal axis shows hour, and a vertical axis showsluminance. The luminance is represented its relative value to initialluminance based on the initial value of 100. The variation of luminanceis measured by applying continuously a constant amount of current to alight-emitting element, that is, applying a stress to a light-emittingelement; and the luminance of the light-emitting element is measured atgiven time intervals. A current that has a current density required toobtain electroluminescence with a luminance of 3000 cd/m² at an initialstate is applied, and the luminance obtained at the applied current ismeasured. FIG. 11 shows that after 100 hours the luminance is decreasedby 14% or less to the initial luminance in each the light-emittingelement (1), (2), (3), and (4). Hence, the luminance of a light-emittingelement practiced by present invention is hardly deteriorated.

Example 7

A method for manufacturing a light-emitting element according to thepresent invention and device characteristics of the light-emittingelement are explained in Example 7. In Example 7, nine light-emittingelements, that is, a light-emitting element (11), a light-emittingelement (12), a light-emitting element (13), a light-emitting element(14), a light-emitting element (15), a light-emitting element (16), alight-emitting element (17), a light-emitting element (18), and alight-emitting element (19), all of which have first electrodes formedby different materials, are manufactured.

A first electrode for the light-emitting element (11) is formed byaluminum containing several % silicon over a glass substrate. A firstelectrode for the light-emitting element (12) is formed by aluminumcontaining several % titanium over a glass substrate. A first electrodeof the light-emitting element (13) is formed by titanium over a glasssubstrate. A first electrode of the light-emitting element (14) isformed by titanium nitride over a glass substrate. A first electrode ofthe light-emitting element (15) is formed by tantalum over a glasssubstrate. A first electrode of the light-emitting element (16) isformed by tantalum nitride over a glass substrate. A first electrode ofthe light-emitting element (17) is formed by tungsten over a glasssubstrate. A first electrode of the light-emitting element (18) isformed by chromium over a glass substrate. A first electrode of thelight-emitting element (19) is formed by molybdenum over a glasssubstrate.

Then, the glass substrate provided with the first electrode is treatedin vacuum at 150° C. for 30 minutes.

A first layer is formed by co-evaporation of a molybdenum oxide,rubrene, and α-NPD over the first electrode to have a weight ratio of0.1:1:0.02, respectively. The first layer is formed to have thickness of60 nm.

A second layer is formed by vapor deposition of α-NPD over the firstlayer to have a thickness of 10 nm.

A third layer is formed by co-evaporation of Alq₃ and coumarin 6 overthe second layer to have a weight ratio of 1:0.005. The third layer isformed to have a thickness of 40 nm.

A fourth layer is formed by vapor deposition of Alq₃ over third layer tohave a thickness of 20 nm.

A fifth layer is formed by co-evaporation of lithium (Li) and4,4′-bis(5-methylbenzoxazole-2-yl)stilbene (abbreviated BzOs) over thefourth layer to have a weight ratio of 0.02:1, respectively. The fifthlayer is formed to have a thickness of 20 nm.

A second electrode is formed by an indium tin oxide (ITO) over the fifthlayer to have a thickness of 110 nm.

To measure an onset voltage for electroluminescence of 1 cd/m² or more,voltage is applied to each of thus manufactured light-emitting elementso that an electric potential of the first electrode is higher than thatof the second electrode. FIG. 12 shows the measurement results indicatedby closed squares. In FIG. 12, a horizontal axis shows the manufacturedlight-emitting elements, and a vertical axis shows voltage (V).

Comparative Example 2

As a comparative example for the light-emitting elements described inExample 7, a light-emitting element (21), a light-emitting element (22),a light-emitting element (23), a light-emitting element (24), alight-emitting element (25), a light-emitting element (26), alight-emitting element (27), a light-emitting element (28), and alight-emitting element (29) are explained hereinafter.

Each layer composing the light-emitting elements (21) to (29) ismanufactured by the same material to have the same thickness as those ofthe light-emitting elements (11) to (19) except that the first layer ofeach the light-emitting elements (21) to (29) is formed by copperphthalocyanine. A first electrode of the light-emitting element (21) isformed by aluminum containing several % silicon. A first electrode ofthe light-emitting element (22) is formed by aluminum containing several% titanium. A first electrode of the light-emitting element (23) isformed by titanium. A first electrode of the light-emitting element (24)is formed by titanium nitride. A first electrode of the light-emittingelement (25) is formed by tantalum. A first electrode of thelight-emitting element (26) is formed by tantalum nitride. A firstelectrode of the light-emitting element (27) is formed by tungsten. Afirst electrode of the light-emitting element (28) is formed bychromium. A first electrode of the light-emitting element (29) is formedby molybdenum.

To measure an onset voltage for electroluminescence of 1 cd/m² or more,voltage is applied to each of thus manufactured light-emitting elementso that an electric potential of the first electrode is higher than thatof the second electrode. FIG. 12 shows the measurement results indicatedby closed triangles.

FIG. 12 shows that the onset voltages of the light-emitting elements(21) to (29) having first layers formed by copper phthalocyanine aredifferent from each other, that is, the onset voltages depend on thematerial for forming the first electrode. On the other hand, the onsetvoltages of the light-emitting elements (11) to (21) practiced by thepresent invention are almost the same with each other despite a materialfor forming the first electrode. The light-emitting element according tothe present invention is less subject to kinds of materials for formingan electrode or the like. Therefore, it becomes easy to select anelectrode formed by a material having better reflectivity in the casethat a light-emitting element in which an electrode is used to reflectlight emission is manufactured according to the present invention.

Example 8

A method for manufacturing a light-emitting element according to thepresent invention and characteristics of the light-emitting element areexplained hereinafter. In Example 8, light-emitting elements, all ofwhich have first layers with different thicknesses, are manufactured.The film thickness dependence of a driving voltage of the light-emittingelements will be revealed.

The light-emitting elements (1) to (4) manufactured in Example 6 areused as samples. The first layer in the light-emitting element (1) isformed to have a thickness of 40 nm. The first layer in thelight-emitting element (2) is formed to have a thickness of 80 nm. Thefirst layer in the light-emitting element (3) is formed to have athickness of 120 nm. The first layer in the light-emitting element (4)is formed to have a thickness of 160 nm.

Voltage is applied to each of the light-emitting elements so that anelectric potential of a first electrode is higher than that of a secondelectrode. FIG. 13 shows current density-voltage characteristics of thelight-emitting elements (1) to (4). The current density-voltagecharacteristics of the light-emitting elements (1) to (4) are hardlydifferent from each other despite the difference in the thicknesses ofthe first layers. Therefore, the increase of a thickness of the firstlayer that is formed by a molybdenum oxide, rubrene, and DNTPD does notresult in the increase of a driving voltage.

Comparative Example 3

As a comparative example for the light-emitting elements described inExample 8, a light-emitting element (30), a light-emitting element (31),a light-emitting element (32), a light-emitting element (33), alight-emitting element (34), and a light-emitting element (35) areexplained hereinafter.

The light-emitting elements (30) to (35), all of which have secondlayers with different thicknesses, are manufactured. A first layer isformed by copper phthalocyanine to have a thickness of 20 nm. The secondlayer is formed by α-NPD by vapor deposition. The second layer of thelight-emitting element (30) is formed to have a thickness of 60 nm. Thesecond layer of the light-emitting element (31) is formed to have athickness of 80 nm. The second layer of the light-emitting element (32)is formed to have a thickness of 100 nm. The second layer of thelight-emitting element (33) is formed to have a thickness of 120 nm. Thesecond layer of the light-emitting element (34) is formed to have athickness of 140 nm. The second layer of the light-emitting element (35)is formed to have a thickness of 160 nm.

Then, a third layer is formed by co-evaporation of Alq₃ and coumarin-6over a second layer to have a weight ratio of 1:0.005. The third layeris formed to have a thickness of 37.5 nm. A fourth layer is formed byvapor deposition of Alq₃ to have a thickness of 37.5 nm over the thirdlayer. A fifth layer is formed by vapor deposition of calcium fluoride(CaF₂) to have a thickness of 1 nm over the fourth layer. A secondelectrode is formed by vapor deposition of aluminum to have a thicknessof 200 nm over the fifth layer.

To operate each of the light-emitting elements, voltage is appliedthereto so that an electric potential of the first electrode is higherthan that of the second electrode. FIG. 14 shows current density-voltagecharacteristics of the light-emitting elements (30) to (35). The drivingvoltages of the light-emitting elements are increased with increasingthe thicknesses of the second layers. Therefore, the increase of athickness of the second layer formed by α-NPD results in the increase ofthe driving voltage.

Comparative Example 4

As a comparative example for the light-emitting elements described inExample 8, a light-emitting element (36), a light-emitting element (37),and a light-emitting element (38) are explained hereinafter. Each offirst layers is formed by a molybdenum oxide to have a differentthickness form each other. A second layer is formed by copperphthalocyanine to have a thickness of 20 nm. A third layer is formed byvapor deposition of α-NPD to have a thickness of 40 nm.

Then, a fourth layer is formed by co-evaporation of Alq₃ and coumarin-6to have a thickness of 37.5 nm over the third layer. A fifth layer isformed by vapor deposition of Alq₃ to have a thickness of 37.5 nm overthe fourth layer. A sixth layer is formed by vapor deposition of CaF₂ tohave a thickness of 1 nm over the fifth layer. A second electrode isformed by vapor deposition of aluminum to have a thickness of 200 nmover the sixth layer.

To operate each of the light-emitting elements, voltage is appliedthereto so that an electric potential of the first electrode is higherthan that of the second electrode. FIG. 15 shows current density-voltagecharacteristics of the light-emitting elements (36) to (38). The drivingvoltages of the light-emitting elements are increased with increasingthe thicknesses of the first layers formed by molybdenum oxides.

Table 1 shows the results of film thickness dependence of drivingvoltage provided by Example 8, Comparative Example 3, and ComparativeExample 4. Table 1 shows the data of a driving voltage required to applya current of 100 mA/cm² to a light-emitting element.

TABLE 1 Required Driving Voltage [V] for Applying 100 mA/cm² to SampleThickness Example Comparative Comparative [nm] 8 Table 3 Table 4 20 — —11.7 40 6.1 — — 50 — — 11.9 60 — 12.5 — 80 6.3 13.5 — 100 — 15.3 12.7120 6.3 16.5 — 140 — 18.9 — 160 6.3 19.9 —

In Example 8, a driving voltage of 6.1 to 6.3 V is required to apply acurrent of 100 mA/cm² to the light-emitting elements (1) to (4). On theother hand, in Comparative Example 3, a driving voltage of 12.5 to 19.9V is required to apply a current of 100 mA/cm² to the light-emittingelements (30) to (35). In Comparative Example 4, a driving voltage of11.7 to 12.7 V is required to apply a current of 100 mA/cm² to thelight-emitting elements (36) to (38).

The results illustrated in Table 1 shows that a driving voltage of alight-emitting element provided with a layer formed by a mixture of anorganic compound and an inorganic can be lower than that of alight-emitting element provided with a layer formed by α-NPD or amolybdenum oxide that is an inorganic compound. Moreover, it also showsthat the increase of the driving voltage can be prevented even if thethickness of the layer is increased.

Therefore, a driving voltage can be reduced by using the light-emittingelement according to the present invention, accordingly, the powerconsumption of the light-emitting element can be reduced. Further, athickness of a light-emitting element can be increased, and so itbecomes possible to reduce short-circuit deterioration between a pair ofelectrodes.

Example 9

Characteristics of a QVGA active matrix light-emitting device having ascreen size of 2.4 inches are explained in Example 9. The active matrixlight-emitting device drives a light-emitting element by a transistor asin the case with Example 4. The light-emitting device has theconfiguration explained in the following.

A first electrode is formed by an indium tin oxide containing a siliconoxide. A first layer is formed by co-evaporation of a molybdenum oxideand α-NPD having high hole transportation properties over the firstelectrode.

A light-emitting device in which the first layer is formed by a mixtureof a molybdenum oxide and α-NPD to have a thickness of 120 nm isreferred to as a light-emitting device 1. Similarly, a light-emittingdevice in which the first layer is formed to have a thickness of 240 nmis referred to as a light-emitting device 2.

A second layer is formed by vapor deposition of α-NPD to have athickness of 10 nm over the first layer. A third layer is formed byco-evaporation of Alq₃ and coumarin-6 to have a thickness of 40 nm overthe second layer.

A fourth layer is formed by vapor deposition of Alq₃ to have a thicknessof 30 nm over the third layer. A fifth layer is formed by vapordeposition of calcium fluoride (CaF₂) to have a thickness of 1 nm overthe fourth layer. A second electrode is formed by vapor deposition ofaluminum to have a thickness of 200 nm over the fifth layer.

The number of dark spots (pixels that do not emit light) in thelight-emitting devices 1 and 2 is checked at the beginning of electricalconduction and after a temperature cycle test (+85° C. (four hours) to−40° C. (four hours)) for 60 hours.

The light-emitting device 1 has the average of the number of dark spotsof 0.7 at the beginning of electrical conduction, and 2.3 after thetemperature cycle test. The light-emitting device 2 has the average ofthe number of dark spots of 0.5 at the beginning of electricalconduction, and 0.5 after the temperature cycle test. As used herein,the term “average of the number of dark spots” refers to an averagevalue in examined closed triangle numbers of light-emitting device.

Comparative Example 5

As a comparative example for Example 9, an active matrix light-emittingdevice 3 in which a light-emitting element has a different configurationis manufactured.

The light-emitting element is formed in the following manner. A firstlayer is formed by copper phthalocyanine to have a thickness of 20 nm. Asecond layer is formed by α-NPD to have a thickness of 40 nm. A thirdlayer is formed by co-evaporation of Alq₃ and coumarin 6 to have athickness of 40 nm over the second layer. A fourth layer is formed byvapor deposition of Alq₃ to have a thickness of 40 nm over the thirdlayer. A fifth layer is formed by vapor deposition of calcium fluoride(CaF₂) to have a thickness of 1 nm over the fourth layer. A secondelectrode is formed by vapor deposition of aluminum to have a thicknessof 200 nm over the fifth layer.

The number of dark spots in the light-emitting device 3 is checked atthe beginning of electrical conduction and after a temperature cycletest (+85° C. (four hours) to −40° C. (four hours)) for 60 hours.

The light-emitting device 3 has the average of the number of dark spotsof 18 at the beginning of electrical conduction, and 444 after thetemperature cycle test. The average of the number of dark spots afterthe temperature cycle test is increased by 25 times as many as that atthe beginning of electrical conduction.

Table 2 illustrates the results of the number of dark spots after thetemperature cycle test (+85° C. (four hours) to −40° C. (four hours))for 60 hours in Example 9 and Comparative Example 5.

TABLE 2 Number of Dark Spot Pixels Beginning of After TemperatureElectrical Conduction Cycle Test Light-emitting Device (1) 0.7 23Light-emitting Device (2) 0.5 0.5 Light-emitting Device (3) 18 444

Dark spots are hardly observed in the light-emitting devices 1 and 2according to Example 9 at the beginning of electrical conduction, andare not increased after a temperature cycle test. On the other hand, agreat number of dark spots are observed in the light-emitting device 3according to Comparative Example 5 at the beginning of electricalconduction, and increased by 25 times after the temperature cycle testas many as those at the beginning of electrical conduction. Thedifference in the results may be resulted from the fact that thethickness of the first layer formed by a mixture of a molybdenum oxideand α-NPD is increased.

The result from Example 9 shows that the number of dark pixel defectscan be drastically reduced by increasing the thickness of alight-emitting element. According to the result from Example 8, it isclear that a driving voltage is not increased with increasing thethickness of a light-emitting element. Therefore, a light-emittingdevice with a low driving voltage and with drastically inhibited darkpixel defects can be provided according to the present invention.

Example 10

Characteristics of a film formed by a molybdenum oxide that is a metaloxide, a film formed by α-NPD that is an organic compound with high holetransporting properties, and a film formed by a mixture of a molybdenumoxide and α-NPD are examined. These films are formed by vapordeposition.

As illustrated in Table 3, the ionization potential of the film formedby a mixture of a molybdenum oxide and α-NPD is approximately 0.1 to 0.2eV less than that of the film formed by a molybdenum oxide and the filmformed by α-NPD. Consequently, hole injection properties of the filmformed by a mixture of a molybdenum oxide and α-NPD are improved.

TABLE 3 Material of Film IP^(b) (eV) MoO₃ −5.48 NPB −5.38 MoO₃—NPB(1:1)^(a) −5.37 MoO₃—NPB (l:0.5)^(a) −5.27 ^(a)mol/mol. ^(b)IonizationPotential (Surveyed Value by AC-2)

FIG. 16 shows absorption spectra of these films. In the absorptionspectra, the spectra of the film of a molybdenum oxide and the film ofα-NPD have no characteristic peak in a visible light region. On theother hand, the absorption of the film formed by a mixture of amolybdenum oxide and α-NPD (OMOx) is less than that of the film formedby a molybdenum oxide. Hence, absorption loss of light can be reduced byusing a layer formed by a mixture of a molybdenum oxide and α-NPD ratherthan using a layer formed by a molybdenum oxide.

In FIG. 16, the spectrum of the film formed by a mixture of a molybdenumoxide and α-NPD has newly an absorption peak at around 500 nm. Theabsorption peak may be resulted from the formation of a charge-transfercomplex between the molybdenum oxide and the α-NPD in the film. Themolybdenum oxide is an accepter, while the α-NPD is a donor. Further, ithas been confirmed that not only α-NPD but also an amine-based compoundsuch as DNTPD serves as a donor.

As described in Example 1 or 2, the lifetime can be extended of alight-emitting element that includes a layer containing a compoundselected from the group of metal oxides and a compound having high holetransporting properties since the crystallization of the layer can besuppressed. As mentioned above, the mixing of an inorganic material andan organic material can generate a synergistic effect that cannot beobtained by using these materials separately.

Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention hereinafterdescribed, they should be construed as being included therein.

1. A method for manufacturing a light-emitting element, the methodcomprising depositing a layer comprising a compound selected from thegroup consisting of oxide semiconductor and a metal oxide, and anorganic compound simultaneously over an electrode.
 2. The methodaccording to claim 1, wherein the compound is selected from molybdenumoxide, vanadium oxide, ruthenium oxide, and tungsten oxide.
 3. Themethod according to claim 1, wherein the compound is molybdenum oxide.4. The method according to claim 1, wherein the organic compound is anaromatic amine.
 5. The method according to claim 1, wherein thedeposition is performed so that the layer is formed to be in contactwith the electrode.
 6. The method according to claim 1, wherein theelectrode is a cathode.
 7. A method for manufacturing a light-emittingelement, the method comprising co-evaporating a compound selected fromthe group consisting of oxide semiconductor and a metal oxide, and anorganic compound to form a layer comprising the compound and the organiccompound over an electrode.
 8. The method according to claim 7, whereinthe compound is selected from molybdenum oxide, vanadium oxide,ruthenium oxide, and tungsten oxide.
 9. The method according to claim 7,wherein the compound is molybdenum oxide.
 10. The method according toclaim 7, wherein the organic compound is an aromatic amine.
 11. Themethod according to claim 7, wherein the evaporation is performed sothat the layer is formed to be in contact with the electrode.
 12. Themethod according to claim 7, wherein the electrode is a cathode.
 13. Amethod for preparing a composite, the method comprising a step ofco-evaporating a compound selected from the group consisting of an oxidesemiconductor and a metal oxide with an organic compound.
 14. The methodaccording to claim 13, wherein the compound is selected from molybdenumoxide, vanadium oxide, ruthenium oxide, and tungsten oxide.
 15. Themethod according to claim 13, wherein the compound is molybdenum oxide.16. The method according to claim 13, wherein the organic compound is anaromatic amine.
 17. The method according to claim 13, wherein theco-evaporation is performed so that the composite is formed as a filmhaving a thickness equal to or larger than 40 nm.